Spectroscopic Signature of Alfvén Waves Damping in a Polar Coronal Hole up to 0.4 Solar Radii

Between 2009 February 24 and 25, the EUV Imaging Spectrometer (EIS) spectrometer on board the Hinode spacecraft performed special "sit and stare" observations above the south polar coronal hole continuously over more than 22 hr. Spectra were acquired with the 1'' slit placed off-limb covering altitudes up to 0.48 R _⊙ (3.34 × 10² Mm) above the Sun surface, in order to study with EIS the non-thermal spectral line broadenings. Spectral lines such as Fe XII λ186.88, Fe XII λ193.51, Fe XII λ195.12, and Fe XIII λ202.04 are observed with good statistics up to high altitudes and they have been analyzed in this study. Results show that the FWHM of the Fe XII λ195.12 line increases up to ~= 0.14 R _⊙, then decreases higher up. EIS stray light has been estimated and removed. Derived electron density and non-thermal velocity profiles have been used to estimate the total energy flux transported by Alfvén waves off-limb in the polar coronal hole up to ~= 0.4 R _⊙. The computed Alfvén wave energy flux density f_w progressively decays with altitude from f_w ~= 1.2 × 10⁶ erg cm^-2 s^-1 at 0.03 R _⊙ down to f_w ~= 8.5 × 10³ erg cm^-2 s^-1 at 0.4 R _⊙, with an average energy decay rate of Δf_w /Δh ~= -4.5 × 10^-5 erg cm^-1. Hence, this result suggests energy deposition by Alfvén waves in a polar coronal hole, thus providing a significant source for coronal heating.